Protection switching
Protection switching (or more generally protection ) describes the possibility in communication networks of automatically switching over to redundant resources in the event of a fault . The communication path (line, connection) normally used is usually referred to as the work path , the redundant one as an alternative path . If it is not a matter of communication channels but, for example, plug-in cards in a device, one speaks of a protected unit and a protective unit.
In the case of communication paths, the mechanisms for recognizing the malfunction and for performing the switching process are implemented at the end points of the protected route.
Protection classes
If a substitute route is kept for each commute, this is referred to as “1 + 1” or “1: 1” protection. With 1 + 1 protection, the sent signal is transmitted simultaneously on both paths and switched over in the event of an error on the receiving side. In contrast to this, with 1: 1 protection, only one path is used at a time, so that the substitute path can be operated with low-priority traffic if there is no error (this is discarded without replacement in the event of an error, “squelching”). If only 1 alternative route is kept for a total of n work routes, this is referred to as “1: n protection” . If m substitute routes are set up for n work routes, one speaks of "m: n protection" .
The operating mode of a protection is revertive or nonrevertive : In revertive mode, after the interrupted work path is restored, it is automatically switched back to it, whereas in nonrevertive mode the substitute path is retained. In this operating mode, you have to switch back manually. A waiting time can usually be set for the automatic switch back (WTR, Wait To Restore ).
Automatic protection switching
Automatic Protection Switching is an automatic security function of SDH systems that allows you to switch from a work route to an alternative route within a very short time in the event of an error. This switchover is usually completed after a maximum of 50 ms, the exact time depending on the technology of the device. It lies in time ranges that older technologies cannot implement.
self-healing
Special protection architectures are called self-healing: This term is used in connection with arrangements that normally have duplicated structures. Examples are SDH ring structures, the double bus structure of the DQDB MAN and the double ring structure of the FDDI LAN.
Equipment protection
While protection is usually used to protect against errors on the transmission paths, equipment protection describes protection against the failure of individual modules or functions on the modules of a device. The protection is created by duplication: Redundant modules or redundant functions must be available that take over protection functions. Equipment Protection increases the reliability of a device. For example, it is common in public exchanges that the central control of the line cards that control the subscriber lines can be duplicated.